Dual conversion gain circuitry with buried channels

ABSTRACT

An imaging device may have an array of image sensor pixels each having a photodiode and a floating diffusion node. Each image sensor pixel in the array may also include a dual conversion gain switch and a dual conversion gain capacitor that allows the image sensor pixel to operate in a low conversion gain mode during which the switch is turned on to share charge between the floating diffusion node and the dual conversion gain capacitor, and a high conversion gain mode in which the switch is turned off. During integration, the photodiode may generate more charge than can be held at the floating diffusion node. A buried channel may be provided beneath the dual conversion gain switch to provide a path along which the excess charge can be shared between the floating diffusion node and the dual conversion gain capacitor even when the dual conversion gain switch is off.

BACKGROUND

This relates to imaging devices, and more particularly, to image sensorpixels having dual conversion gain functionality.

Image sensors are commonly used in electronic devices such as cellulartelephones, cameras, and computers to capture images. In a typicalarrangement, an image sensor includes an array of image pixels arrangedin pixel rows and pixel columns. Circuitry may be coupled to each pixelcolumn for reading out image signals from the image pixels.

Typical image pixels include a photodiode for generating charge inresponse to incident light. Image pixels may include a dual conversiongain switch that allows the pixel to operate in a high conversion gainmode in low light conditions in which increased sensitivity is desired,and to operate in a low conversion gain mode in high light conditions inwhich increased charge storage capacity is desired. In some instances,it may be desirable to use the increased charge storage capacityprovided by the dual conversion gain circuitry even when the pixel isbeing operated in the high conversion gain mode. While the dualconversion gain switch can be turned on to access the increased chargestorage capacity of the pixel, this may increase the dark current andnoise associated with the pixel.

It would therefore be desirable to be able to provide imaging deviceswith improved image sensor pixels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an illustrative electronic device having an imagesensor and processing circuitry for capturing images using an array ofimage pixels in accordance with an embodiment.

FIG. 2 is a diagram of an illustrative pixel array and associatedreadout circuitry for reading out image signals from the pixel array inaccordance with an embodiment.

FIG. 3 is a circuit diagram of an illustrative image sensor pixel havingdual conversion gain circuitry in accordance with an embodiment.

FIG. 4 is a cross-sectional side view of a portion of an illustrativeimage sensor pixel having a dual conversion gain switch with a buriedchannel in accordance with an embodiment.

FIG. 5 is a potential diagram of an integration period for anillustrative image sensor pixel having a dual conversion gain switchwith a buried channel in accordance with an embodiment.

FIG. 6 is a timing diagram for sampling charge from an illustrativeimage sensor pixel having a dual conversion gain switch with a buriedchannel in accordance with an embodiment.

DETAILED DESCRIPTION

Embodiments of the present invention relate to image sensors, and moreparticularly, to image sensors having pixels with dual conversion gain(DCG) functionality. It will be recognized by one skilled in the artthat the present exemplary embodiments may be practiced without some orall of these specific details. In other instances, well known operationshave not been described in detail in order to not unnecessarily obscurethe present embodiments.

Imaging systems having digital camera modules are widely used inelectronic devices such as digital cameras, computers, cellulartelephones, and other electronic devices. A digital camera module mayinclude one or more image sensors that gather incoming light to capturean image. Image sensors may include arrays of image pixels. The pixelsin the image sensors may include photosensitive elements such asphotodiodes that convert the incoming light into electric charge. Imagesensors may have any number of pixels (e.g., hundreds or thousands ormore). A typical image sensor may, for example, have hundreds,thousands, or millions of pixels (e.g., megapixels). Image sensors mayinclude control circuitry such as circuitry for operating the imagepixels and readout circuitry for reading out image signals correspondingto the electric charge generated by the photosensitive elements.

FIG. 1 is a diagram of an illustrative imaging system such as anelectronic device that uses an image sensor to capture images.Electronic device 10 of FIG. 1 may be a portable electronic device suchas a camera, a cellular telephone, a tablet computer, a webcam, a videocamera, a video surveillance system, an automotive imaging system, avideo gaming system with imaging capabilities, or any other desiredimaging system or device that captures digital image data. Camera module12 may be used to convert incoming light into digital image data. Cameramodule 12 may include one or more lenses 14 and one or morecorresponding image sensors 16. Lenses 14 may include fixed and/oradjustable lenses and may include microlenses formed on an imagingsurface of image sensor 16. During image capture operations, light froma scene may be focused onto image sensor 16 by lenses 14. Image sensor16 may include circuitry for converting analog pixel data intocorresponding digital image data to be provided to storage andprocessing circuitry 18. If desired, camera module 12 may be providedwith an array of lenses 14 and an array of corresponding image sensors16.

Storage and processing circuitry 18 may include one or more integratedcircuits (e.g., image processing circuits, microprocessors, storagedevices such as random-access memory and non-volatile memory, etc.) andmay be implemented using components that are separate from camera module12 and/or that form part of camera module 12 (e.g., circuits that formpart of an integrated circuit that includes image sensors 16 or anintegrated circuit within module 12 that is associated with imagesensors 16). Image data that has been captured by camera module 12 maybe processed and stored using processing circuitry 18 (e.g., using animage processing engine on processing circuitry 18, using an imagingmode selection engine on processing circuitry 18, etc.). Processed imagedata may, if desired, be provided to external equipment (e.g., acomputer, external display, or other device) using wired and/or wirelesscommunications paths coupled to processing circuitry 18.

As shown in FIG. 2, image sensor 16 may include a pixel array 20containing image sensor pixels 22 arranged in rows and columns(sometimes referred to herein as image pixels or pixels) and control andprocessing circuitry 24. Array 20 may contain, for example, hundreds orthousands of rows and columns of image sensor pixels 22. Controlcircuitry 24 may be coupled to row control circuitry 26 and imagereadout circuitry 28 (sometimes referred to as column control circuitry,readout circuitry, processing circuitry, or column decoder circuitry).Row control circuitry 26 may receive row addresses from controlcircuitry 24 and supply corresponding row control signals such as reset,row-select, charge transfer, dual conversion gain, and readout controlsignals to pixels 22 over row control paths 30. One or more conductivelines such as column lines 32 may be coupled to each column of pixels 22in array 20. Column lines 32 may be used for reading out image signalsfrom pixels 22 and for supplying bias signals (e.g., bias currents orbias voltages) to pixels 22. If desired, during pixel readoutoperations, a pixel row in array 20 may be selected using row controlcircuitry 26 and image signals generated by image pixels 22 in thatpixel row can be read out along column lines 32.

Image readout circuitry 28 (sometimes referred to as column readout andcontrol circuitry 28) may receive image signals (e.g., analog pixelvalues generated by pixels 22) over column lines 32. Image readoutcircuitry 28 may include sample-and-hold circuitry for sampling andtemporarily storing image signals read out from array 20, amplifiercircuitry, analog-to-digital conversion (ADC) circuitry, bias circuitry,column memory, latch circuitry for selectively enabling or disabling thecolumn circuitry, or other circuitry that is coupled to one or morecolumns of pixels in array 20 for operating pixels 22 and for readingout image signals from pixels 22. ADC circuitry in readout circuitry 28may convert analog pixel values received from array 20 intocorresponding digital pixel values (sometimes referred to as digitalimage data or digital pixel data). Image readout circuitry 28 may supplydigital pixel data to control and processing circuitry 24 and/orprocessor 18 (FIG. 1) over path 25 for pixels in one or more pixelcolumns.

If desired, image pixels 22 may include one or more photosensitiveregions for generating charge in response to image light. Photosensitiveregions within image pixels 22 may be arranged in rows and columns onarray 20. Pixel array 20 may be provided with a color filter arrayhaving multiple color filter elements which allows a single image sensorto sample light of different colors. As an example, image sensor pixelssuch as the image pixels in array 20 may be provided with a color filterarray which allows a single image sensor to sample red, green, and blue(RGB) light using corresponding red, green, and blue image sensor pixelsarranged in a Bayer mosaic pattern. The Bayer mosaic pattern consists ofa repeating unit cell of two-by-two image pixels, with two green imagepixels diagonally opposite one another and adjacent to a red image pixeldiagonally opposite to a blue image pixel. In another suitable example,the green pixels in a Bayer pattern are replaced by broadband imagepixels having broadband color filter elements (e.g., clear color filterelements, yellow color filter elements, etc.). These examples are merelyillustrative and, in general, color filter elements of any desired colorand in any desired pattern may be formed over any desired number ofimage pixels 22.

A circuit diagram of an illustrative image pixel 22 of image pixel array20 is shown in FIG. 3. As shown in FIG. 3, pixel 22 may include aphotosensitive element such as photodiode (PD) 34 (sometimes referred toherein as photodetector 34). A positive pixel power supply voltage(e.g., voltage Vpix) may be supplied at positive power supply terminal36. A ground power supply voltage may be supplied at ground terminal 38and ground terminal 44. Photodiode 34 may generate charge (e.g.,electrons) in response to receiving impinging photons (i.e., incidentlight). The amount of charge that is collected by photodiode 34 dependson the intensity of the impinging light and the exposure duration(sometimes referred to herein as an integration period integrationtime).

Before the integration period begins, reset control signal RG may beasserted. This turns on (sometimes referred to herein as activating)reset transistor 40 and resets charge storage node 42 (also referred toas floating diffusion FD) to a voltage equal or close to Vpix (a resetvoltage). During this time, transfer gate 46 may also be turned on andoff (transfer signal TG may be asserted and then deasserted) to resetphotodiode 34. The reset control signal RG may then be deasserted toturn off (sometimes referred to herein as deactivating) reset transistor40. After the reset process is complete, the integration period beginsand charge begins to accumulate on photodiode 34 in response to incidentlight. After the integration period has elapsed, transfer gate controlsignal TG may be asserted to turn on transfer transistor (transfer gate)46. When transfer transistor 46 is turned on, the charge that has beengenerated by photodiode 34 in response to the incoming light (i.e.,during the integration period) is transferred to charge storage node 42.

Charge storage node 42 may be implemented using a region of dopedsemiconductor (e.g., a doped silicon region formed in a siliconsubstrate by ion implantation, impurity diffusion, or other dopingtechniques). The doped semiconductor region (i.e., the floatingdiffusion node FD) may exhibit a capacitance that can be used to storethe charge that has been transferred from photodiode 34. The signalassociated with the stored charge on node 42 is buffered bysource-follower (SF) transistor 48. Row select transistor 50 connectsthe source follower transistor 48 to column output line 53 via path 52.

When it is desired to read out the charge stored on floating diffusionnode 42 (i.e., the amount of stored charge that is represented by thesignal at the source of transistor 48), row select control signal SELcan be asserted. When signal SEL is asserted, transistor 50 turns on anda corresponding signal Vout that is representative of the magnitude ofthe charge on charge storage node 42 is produced on output path 52. In atypical configuration, there are numerous rows and columns of pixelssuch as pixel 22 in the image sensor pixel array 20 of a given imagesensor 16. A conductive path such as column readout line 53 can beassociated with one column of image pixels 22. The signal Vout on path52 is coupled onto column readout line 53 for each pixel in array 20 asrow select control signal SEL is asserted in each pixel. Column readoutline 53 routes signal Vout from the pixel 22 to readout circuitry (e.g.,column readout and control circuitry 28 in FIG. 2).

In the arrangement of FIG. 3, pixel 22 is provided with dual conversiongain circuitry including dual conversion gain switch (DCG) 54 (sometimesreferred to herein as a dual conversion gain transistor or a dualconversion gain gate) and dual conversion gain capacitor (DCG Cap) 56.Dual conversion gain circuitry allows pixel 22 to operate in a lowconversion gain mode and a high conversion gain mode.

When operated in the low conversion gain mode, dual conversion gainswitch 54 is turned on. This couples floating diffusion node 42 to dualconversion gain capacitor 56, providing floating diffusion node 42 withadditional capacitance. When the charge generated by photodiode 34 andtransferred to floating diffusion node 42 is too large to be held atfloating diffusion node 42, the excess charge may be stored on capacitor56, thereby increasing the charge holding capacity of the pixel whiledecreasing its sensitivity compared to when dual conversion gain switch54 is off. By leaving dual conversion gain switch 54 on during readoutoperations (e.g., when row select transistor 50 is turned on), thecharge stored on dual conversion gain capacitor 56 may be read out withthe charge stored on floating diffusion node 42. By leaving dualconversion gain switch 54 on during reset operations (e.g., when resettransistor 40 is turned on), dual conversion gain capacitor 56 can bereset to a reset voltage equal to or close to Vpix.

When operated in the high conversion gain mode, dual conversion gainswitch 54 may be turned off. In this case, charge that is generated bythe photodiode 34 is not routed to dual conversion gain capacitor 56,thereby decreasing the charge-holding capacity of the pixel whileincreasing its sensitivity compared to when dual conversion gain switch54 is on. In this case, the charge-holding capacity of the pixel islimited to capacitance of the floating diffusion node 42.

In some scenarios, it may be desirable to use the excess charge storagecapabilities of dual conversion gain capacitor 56 even when pixel 22 isoperating in the high conversion gain mode. For example, if the chargeholding capacity of photodiode 34 is exceeded during the integrationperiod (during high illumination conditions, for example), some of thecharge may overflow through transfer gate 46 to be held at floatingdiffusion node 42 (i.e., charge may flow through gate 46 to node 42 eventhough gate 46 is off). In order to increase the overflow capacity ofthe pixel 22 in scenarios such as this, it may be desirable to use theexcess charge storage capabilities of dual conversion gain capacitor 56to store the overflow charge.

One way to utilize dual conversion gain capacitor 56 to store overflowcharge during integration is to periodically turn on (sometimes referredto herein as pulsing or toggling) dual conversion gain switch 54 duringintegration, thereby allowing excess charge on floating diffusion node42 to be shared with (transferred to) dual conversion gain capacitor 56.Toggling dual conversion gain switch in this way, however, may generateadditional dark current from floating diffusion node 42. This increaseddark current may increase the noise floor of the pixel 22 or increasethe fixed pattern noise associated with the pixel 22, potentiallydecreasing the quality of the images produced.

In order to allow dual conversion gain capacitor 56 to store overflowcharge during integration without pulsing dual conversion gain switch54, pixel 22 may be provided with a buried channel beneath switch 54that creates a path along which charge can move from floating diffusionnode 42 to dual conversion gain capacitor 56 without turning dualconversion gain switch 54 on. A cross-sectional side view of a portionof an illustrative image sensor pixel 22 having a dual conversion gainswitch 54 with a buried channel is shown in FIG. 4. In the portion ofpixel 22 shown in FIG. 4, semiconductor substrate 58 includes a portionof floating diffusion node 42 and a portion of dual conversion gaincapacitor 56 formed therein. If desired, the portions of floatingdiffusion node 42 and dual conversion gain capacitor 56 may be N+-typedoped portions (sometimes referred to herein as doped regions) ofsemiconductor substrate 58. A doped portion 60 (sometimes referred toherein as doped region 60) of semiconductor substrate 58 may extend fromfloating diffusion node 42 to dual conversion gain capacitor 56, formingan electrical path between node 42 and capacitor 56. As shown in FIG. 4,doped portion 60 may be formed at the surface of semiconductor substrate58. If desired, doped portion 60 may be a P-type doped portion ofsemiconductor substrate 58. Dual conversion gain gate 54 is formed overdoped portion 60, with gate oxide 64 between gate 54 and doped portion60.

In addition to doped portion 60, a buried channel 68 may also be formedin semiconductor substrate 58. Buried channel 68 may extend fromfloating diffusion node 42 to dual conversion gain capacitor 56, formingan electrical path between the node 42 and channel capacitor 56. Ifdesired, buried channel 68 may be an N-type doped portion ofsemiconductor substrate 58 (sometimes referred to herein as a dopedregion). As shown in FIG. 4, channel 68 is “buried” in semiconductorsubstrate 58 (i.e., formed below the surface of semiconductor substrate58 and/or embedded within semiconductor substrate 58) below dopedportion 60.

When dual conversion gain gate 54 is turned on and pixel 22 is operatingin the low conversion gain mode, charge will flow from floatingdiffusion node 42 to dual conversion gain capacitor 56 along path 62(i.e., through doped portion 60). When dual conversion gain gate 54 isturned off and pixel 22 is operating in high conversion gain mode,however, some charge will still flow from floating diffusion node 42 todual conversion gain capacitor 56. In this instance, however, the chargewill flow along path 70 (i.e., through buried channel 68). In this way,buried channel 68 allows dual conversion gain capacitor 56 to be usedfor overflow charge storage even when dual conversion gain gate 54 isoff and pixel 22 is operating in high conversion gain mode.

FIG. 5 shows a potential diagram for an integration period of anillustrative image sensor image pixel 22 of the type described above inconnection with FIG. 4. During the integration period, transfer gate 46may be turned off (transfer gate signal TG may be deasserted such thattransfer gate 46 is held at voltage V_(TG_low)). As photodiode 34 (PD)generates electrical charge in response to incident light, the chargeholding capacity of photodiode 34 may be exceeded (i.e., the potentialof photodiode 34 may reach or drop below V_(TG_low)), causing electricalcharge to overflow from photodiode 34 into floating diffusion 42 (FD),as shown by arrow 72.

In the illustrative example of FIG. 5, dual conversion gain switch 54 isoff and pixel 22 is operating in high conversion gain mode. In theabsence of buried channel 68, the potential barrier between floatingdiffusion node 42 and dual conversion gain capacitor 56 would beapproximately equal to V_(TG_low) (as shown by the dotted line in FIG.5). As the overflow charge accumulates on floating diffusion node 42,the charge holding capacity of floating diffusion node 42 may beexceeded (i.e., the potential of floating diffusion node 42 may reach ordrop below V_(TG_low)). In this instance, the potential barrier betweenfloating diffusion node 42 and dual conversion gain capacitor 56 is thesame as the potential barrier between floating diffusion node 42 andphotodiode 34. In order to allow the excess charge to move from floatingdiffusion node 42 to dual conversion gain capacitor 56, the bias voltageat dual conversion gain switch 54 would have to be increased (i.e.,switch 54 would have to be turned on). This, however, may generate darkcurrent and the risk of unwanted additional noise in the image signal.

When buried channel 68 is present, however, the potential barrierbetween dual conversion gain capacitor 56 and floating diffusion node 42is decreased to V_(DCG_low) even when dual conversion gain switch 54 isoff. Thus, as the overflow charge accumulates on floating diffusion node42 such that the charge holding capacity of floating diffusion 42 isexceeded (i.e., the potential of floating diffusion node 42 reaches ordrops below V_(DCG_low)), the excess charge may overflow from floatingdiffusion node 42 to dual conversion gain capacitor 56 as shown by arrow76. This allows the overflow charge to be integrated without introducingadditional dark current and noise associated with pulsing dualconversion gain switch 54.

During the integration period, reset gate 40 is turned off (reset gatesignal RG may be deasserted such that reset gate 40 is held at voltageV_(RG_low)). In general, and as shown in FIG. 5, the relationship shownbelow in equation (1) should be maintained between the relativepotentials of transfer gate 46, dual conversion gain switch 54, andreset gate 40 during the integration period:0V<V_(TG_low)<V_(RG_low)<V_(DCG_low)  (1)After the integration period has elapsed and the charge on floatingdiffusion node 42 and dual conversion gain capacitor 56 have been readout, reset gate signal RG may be asserted to reset photodiode 34,floating diffusion node 42, and dual conversion gain capacitor 56 to avoltage at or near Vpix.

A timing diagram for sampling charge from an illustrative image sensorpixel 22 of the type shown and described in connection with FIGS. 3-5 isshown in FIG. 6. At time t1, reset signal RG may be asserted, turningreset transistor 40 on. Reset signal RG may continue to be asserteduntil time t2. Between time t1 and time t2, dual conversion gain signalDCG may be asserted and transfer gate signal TG may be asserted. Thisturns on dual conversion gain switch 54 and transfer gate 46, therebyresetting photodiode 34, floating diffusion node 42, and dual conversiongain capacitor 56 to a reset voltage (e.g., Vpix).

At time t2, reset operations end and the integration period begins.During the integration period, photodiodes 34 generate charge inresponse to incident light. As described above in connection with FIGS.4 and 5, excess charge that overflows from photodiode 34 during theintegration period may be stored on floating diffusion node 42. Due tothe presence of buried channel 68, excess charge that overflows fromfloating diffusion node 42 during the integration period (in highillumination conditions, for example) may be stored on dual conversiongain capacitor 56. As shown in FIG. 6, charge may flow from floatingdiffusion node 42 to dual conversion gain capacitor 56 without assertingdual conversion gain signal DCG (i.e., without turning on dualconversion gain switch 54) during the integration period.

Near the end of the integration period, image readout circuitry 28,including its sample-and-hold circuitry, may begin to sample the imagesignals generated by pixel 22. At time t3, row select control signal SELis asserted, allowing the charge generated by photodiode 34 to be readout. At time t4, dual conversion gain signal DCG is asserted, turningdual conversion gain switch 54 on and ending the integration period(i.e., the integration period lasts from t2 to t4). When the DCG signalis asserted, the overflow charge accumulated at both floating diffusionnode 42 and dual conversion gain capacitor 56 during the integrationperiod, (but not the charge generated by photodiode 34 that did notoverflow) is coupled onto source-follower transistor 48 and coupled ontocolumn output line 53 via path 52 (read out) as sample signal SHS_DCG+FD(sometimes referred to herein as a first sample signal).

At time t5, and while signal DCG is still asserted, reset signal RG isasserted, resetting floating diffusion node 42 and dual conversion gaincapacitor 56 to a reset voltage (e.g., Vpix). Signal DCG is thendeasserted, turning off dual conversion gain capacitor 56. After DCG isdeasserted, reset signal RG is deasserted. The reset voltage on floatingdiffusion node 42 is then coupled onto source follower transistor 48 andcoupled onto column output line 53 (read out) via path 52 as resetsignal SHR.

At time t6, transfer signal TG is asserted, coupling the chargegenerated by photodiode 34 (that did not overflow) during theintegration period onto floating diffusion node 42. Once the charge hasbeen transferred onto floating diffusion 42, transfer signal TG isdeasserted. The charge on floating diffusion node 42 is coupled ontosource-follower transistor 48 and coupled onto column output line 53(read out) via path 52 as sample signal SHS_PD (sometimes referred toherein as a second or additional sample signal).

At time t7 row select signal SEL is deasserted, ending readoutoperations. Reset signal RG is asserted, resetting floating diffusionnode 42 to a reset voltage (e.g., Vpix).

After signals SHS_DCG+FD, SHR, and SHS_PD have been sampled, processingcircuitry, such as control and processing circuitry 24 of FIG. 2, may beused to generate an image signal indicative of the total amount ofcharge generated by photodiode 34 (including overflow charge). Readingout both the reset signal SHR and the sample signals SHS_DCG+FD andSHS_PD allows for noise and/or dark current compensation to be performedon the signals (e.g., through subtraction operations). These subtractionoperations may be referred to as double sampling. Since reset voltagelevel (SHR) readout occurs after SHS_DCG+FD sample readout, the doublesampling readout for SHS_DCG+FD may be a readout with uncorrelated noise(e.g., the double sampling readout is not a correlated double samplingreadout). However, the double sampling readout may still reduce noiselevels compared to reading out the signal level alone. Since the resetvoltage readout occurs before SHS_PD readout, correlated double samplingmay be used to generate a noise and/or dark current-compensated SHS_PDreadout signal. After being processed through correlated double samplingoperations and/or double sampling operations with uncorrelated noise,signals SHS_DCG+FD and SHS_PD may be combined to generate anoise-corrected signal indicative of the total amount of charge(including overflow charge) generated by photodiode 34 during theintegration time.

Although described above in connection with one pixel 22, each pixel 22in array 20 may be provided with dual conversion gain circuitry (e.g.,switch 54 and capacitor 56) having a buried channel 68 that is capableof generating image signals indicative of the total amount of chargegenerated by photodiode 34 (including overflow charge) with reducednoise. The image signals from each of the pixels 22 in array 20 may thenbe combined (e.g., by processing circuitry 24) to generate a digitalimage.

Various embodiments have been described illustrating image sensor pixelshaving dual conversion gain circuitry with buried channels.

In various embodiments, an image sensor pixel formed on a semiconductorsubstrate may include a photodetector that generates charge in responseto incident light, a floating diffusion node that stores the charge, adual conversion gain capacitor, a dual conversion gain switch that maytransfer the charge from the floating diffusion node to the dualconversion gain capacitor while the dual conversion gain switch is on,and a buried channel in the semiconductor substrate that may transferthe charge from the floating diffusion node to the dual conversion gaincapacitor while the dual conversion gain switch is off.

In some embodiments, the photodetector may generate the charge inresponse to the incident light during an integration period, and thecharge stored on the floating diffusion node may overflow from thephotodetector to the floating diffusion node during the integrationperiod. During the integration period, the charge stored on the floatingdiffusion node may overflow from the floating diffusion node to the dualconversion gain capacitor through the buried channel while the dualconversion gain switch is off.

In some embodiments, the floating diffusion node and the dual conversiongain capacitor may include N+-type doped portions of the semiconductorsubstrate. The buried channel may extend from the floating diffusionnode to the dual conversion gain capacitor. The buried channel mayinclude an N-type doped portion of the semiconductor substrate. Thecharge that is transferred from the floating diffusion node to the dualconversion gain capacitor while the dual conversion gain switch is offmay be transferred through the N-type doped portion. The image sensorpixel may further include a P-type doped portion of the semiconductorsubstrate between the N-type doped portion of the semiconductorsubstrate and the dual conversion gain switch. The charge that istransferred from the floating diffusion node to the dual conversion gaincapacitor while the dual conversion gain switch is on may be transferredthrough the P-type doped portion.

In various embodiments, an image sensor pixel may include a photodiodethat generates charge in response to incident light during anintegration time, and a floating diffusion node. The charge generated bythe photodiode may overflow from the photodiode to the floatingdiffusion node during the integration time. The image sensor pixel mayfurther include a dual conversion gain transistor and a dual conversiongain capacitor. Charge may flow from the floating diffusion node to thedual conversion gain capacitor during the integration time withoutturning on the dual conversion gain transistor.

In some embodiments, the image sensor pixel may be formed in asemiconductor substrate and may further include a buried channel in thesemiconductor substrate. During the integration time, charge may flowfrom the floating diffusion node to the dual conversion gain capacitorthrough the buried channel, without turning on the dual conversion gaintransistor. The image sensor pixel may further include a doped region inthe semiconductor substrate through which, during the integration time,the charge may flow from the floating diffusion node to the dualconversion gain capacitor when the dual conversion gain transistor isturned on. The doped region may be between the buried channel and thedual conversion gain transistor. The floating diffusion node and thedual conversion gain capacitor may be formed in the semiconductorsubstrate. The buried channel may extend from the floating diffusionnode to the dual conversion gain capacitor. The doped region may extendfrom the floating diffusion node to the dual conversion gain capacitor.

In some embodiments, the image sensor pixel may further include atransfer gate coupled between the photodiode and the floating diffusionnode. Charge may overflow from the photodiode to the floating diffusionnode through the transfer gate. The image sensor pixel may furtherinclude a reset gate coupled to the photodiode, the floating diffusionnode, and the dual conversion gain capacitor. During the integrationtime, the dual conversion gain transistor may be held at a firstpotential, the reset gate may be held at a second potential that is lessthan the first potential, and the transfer gate may be held at a thirdpotential that is less than the second potential.

In various embodiments, a method of operating an image sensor pixelhaving a photodiode, a floating diffusion node, a dual conversion gaincapacitor, a dual conversion gain switch, and a buried channel beneaththe dual conversion gain switch may include generating charge with thephotodiode during an integration time. At least some of the charge mayoverflow from the photodiode during the integration time. The chargethat overflows from the photodiode may be stored on the floatingdiffusion node. At least some of the charge may be transferred from thefloating diffusion node to the dual conversion gain capacitor usingburied channel without activating the dual conversion gain switch.

In some embodiments, the image sensor further includes a reset gate anda transfer gate that is coupled between the photodetector and thefloating diffusion node. The method may further include activating thetransfer gate and the dual conversion gain switch, before theintegration time begins and while the reset gate is activated, to resetthe photodiode, the floating diffusion node, and the dual conversiongain capacitor. The image sensor pixel may further include a row selecttransistor. The method may further include activating the dualconversion gain switch, after the integration time has ended and whilethe row select transistor is activated, to read out the charge on thefloating diffusion node and the dual conversion gain capacitor as asample signal. The method may further include activating the reset gateto reset the floating diffusion node and the dual conversion gaincapacitor again after reading out the charge on the floating diffusionnode and the dual conversion gain capacitor. After resetting thefloating diffusion node and the dual conversion gain capacitor again,the dual conversion gain switch may be deactivated. After deactivatingthe dual conversion gain switch, the charge on the floating diffusionnode may be read out as a reset signal. The method may further includeactivating the transfer gate, after reading out the charge on thefloating diffusion node as a reset signal and while the row selecttransistor is turned on, to transfer, to the floating diffusion node,the charge generated by the photodiode that did not overflow during theintegration period. The the charge generated by the photodiode that didnot overflow during the integration period may be read out from thefloating diffusion node as an additional sample signal.

The foregoing is merely illustrative and various modifications can bemade to the described embodiments. The foregoing embodiments may beimplemented individually or in any combination.

What is claimed is:
 1. An image sensor pixel formed on a semiconductorsubstrate, the image sensor pixel comprising: a photodetector thatgenerates charge in response to incident light; a floating diffusionnode that stores the charge; a dual conversion gain capacitor; a dualconversion gain switch that transfers the charge from the floatingdiffusion node to the dual conversion gain capacitor while the dualconversion gain switch is on; and a buried channel in the semiconductorsubstrate that transfers the charge from the floating diffusion node tothe dual conversion gain capacitor while the dual conversion gain switchis off.
 2. The image sensor pixel defined in claim 1 wherein thephotodetector generates the charge in response to the incident lightduring an integration period, and wherein the charge stored on thefloating diffusion node overflows from the photodetector to the floatingdiffusion node during the integration period.
 3. The image sensor pixeldefined in claim 2 wherein, during the integration period, the chargestored on the floating diffusion node overflows from the floatingdiffusion node to the dual conversion gain capacitor through the buriedchannel while the dual conversion gain switch is off.
 4. The imagesensor pixel defined in claim 1 wherein the floating diffusion node andthe dual conversion gain capacitor comprise N+-type doped portions ofthe semiconductor substrate.
 5. The image sensor pixel defined in claim4 wherein the buried channel extends from the floating diffusion node tothe dual conversion gain capacitor.
 6. The image sensor pixel defined inclaim 5 wherein the buried channel comprises an N-type doped portion ofthe semiconductor substrate and wherein the charge that is transferredfrom the floating diffusion node to the dual conversion gain capacitorwhile the dual conversion gain switch is off is transferred through theN-type doped portion.
 7. The image sensor pixel defined in claim 6further comprising: a P-type doped portion of the semiconductorsubstrate interposed between the N-type doped portion of thesemiconductor substrate and the dual conversion gain switch, wherein thecharge that is transferred from the floating diffusion node to the dualconversion gain capacitor while the dual conversion gain switch is on istransferred through the P-type doped portion.
 8. An image sensor pixelcomprising: a photodiode that generates charge in response to incidentlight during an integration time; a floating diffusion node, wherein thecharge overflows from the photodiode to the floating diffusion nodeduring the integration time; a dual conversion gain transistor coupledto the floating diffusion node; and a dual conversion gain capacitor,wherein the dual conversion gain transistor is interposed between thefloating diffusion node and the dual conversion gain capacitor andwherein the charge flows from the floating diffusion node to the dualconversion gain capacitor during the integration time without turning onthe dual conversion gain transistor.
 9. The image sensor pixel definedin claim 8 wherein the image sensor pixel is formed in a semiconductorsubstrate, the image sensor pixel further comprising: a buried channelin the semiconductor substrate through which, during the integrationtime, the charge flows from the floating diffusion node to the dualconversion gain capacitor without turning on the dual conversion gaintransistor.
 10. The image sensor pixel defined in claim 9 furthercomprising: a doped region in the semiconductor substrate through which,during the integration time, the charge flows from the floatingdiffusion node to the dual conversion gain capacitor when the dualconversion gain transistor is turned on, wherein the doped region isinterposed between the buried channel and the dual conversion gaintransistor.
 11. The image sensor pixel defined in claim 10 whereinfloating diffusion node and the dual conversion gain capacitor areformed in the semiconductor substrate.
 12. The image sensor pixeldefined in claim 11 wherein the buried channel extends from the floatingdiffusion node to the dual conversion gain capacitor.
 13. The imagesensor pixel defined in claim 12 wherein the doped region extends fromthe floating diffusion node to the dual conversion gain capacitor. 14.The image sensor pixel defined in claim 8 further comprising: a transfergate coupled between the photodiode and the floating diffusion node,wherein the charge overflows from the photodiode to the floatingdiffusion node through the transfer gate; and a reset gate coupled tothe photodiode, the floating diffusion node, and the dual conversiongain capacitor.
 15. The image sensor pixel defined in claim 14 whereinduring the integration time, the dual conversion gain transistor is heldat a first potential, the reset gate is held at a second potential thatis less than the first potential, and the transfer gate is held at athird potential that is less than the second potential.
 16. A method ofoperating an image sensor pixel having a photodiode, a floatingdiffusion node coupled to the photodiode, a dual conversion gaincapacitor, a dual conversion gain switch that is coupled between thefloating diffusion node and the dual conversion capacitor, and a buriedchannel beneath the dual conversion gain switch, the method comprising:with the photodiode, generating charge during an integration time,wherein at least some of the charge overflows from the photodiode duringthe integration time; with the floating diffusion node, storing thecharge that overflows from the photodiode; and with the buried channel,transferring at least some of the charge from the floating diffusionnode to the dual conversion gain capacitor without activating the dualconversion gain switch.
 17. The method defined in claim 16 wherein theimage sensor further comprises a reset gate and a transfer gate that iscoupled between the photodiode and the floating diffusion node, themethod further comprising: before the integration time begins,activating the transfer gate and the dual conversion gain switch whilethe reset gate is activated to reset the photodiode, the floatingdiffusion node, and the dual conversion gain capacitor.
 18. The methoddefined in claim 17 wherein the image sensor further comprises a rowselect transistor, the method further comprising: after the integrationtime has ended, activating the dual conversion gain switch while the rowselect transistor is activated to read out the charge on the floatingdiffusion node and the dual conversion gain capacitor as a samplesignal; after reading out the charge on the floating diffusion node andthe dual conversion gain capacitor, activating the reset gate to resetthe floating diffusion node and the dual conversion gain capacitoragain; after resetting the floating diffusion node and the dualconversion gain capacitor again, deactivating the dual conversion gainswitch; after deactivating the dual conversion gain switch, reading outthe charge on the floating diffusion node as a reset signal; afterreading out the charge on the floating diffusion node as a reset signal,activating the transfer gate while the row select transistor is turnedon to transfer, to the floating diffusion node, the charge generated bythe photodiode that did not overflow during the integration period; andreading out, from the floating diffusion node, the charge generated bythe photodiode that did not overflow during the integration period as anadditional sample signal.
 19. The image sensor pixel defined in claim 1,wherein the floating diffusion node is coupled to the photodetector andwherein the dual conversion gain switch is coupled between the floatingdiffusion node and the dual conversion gain capacitor.
 20. The imagesensor pixel defined in claim 8, wherein the floating diffusion node iscoupled to the photodiode.